ABSTRACT
With respect to control of spermatogenesis, removal of the ventral lobe in Scyliorhinus canicula caused regression of the testis with the appearance of a localized zone of degeneration between the last generation of spermatogonia and the first appearance of spermatocytes (Dobson and Dodd 1977b, c), suggesting that this transition may be gonadotropin-sensitive. On the other hand, replacement with human chorionic gonadotropin (hCG: LH-like) or pregnant mares serum goadotropin (PMS: FSH-like) did not prevent the effects of hypophysectomy, confirming that putative gonadotrophin receptors in elasmobranchs are unable to recognize heterologous mammalian hormones. ln the same studies, ventral lobectomy reduced [3H]thymidine incorporation into whole testis. Although no attempt was made to determine whether this effect was stage-specific, it was seasonal. Only fish collected in summer (April to September) displayed a zone of degeneration after ventral lobectomy (Dobson and Dodd 1977b, c). ln a further study, Dobson and Dodd showed that temperatures resembling those in nature in summer (10-15°C) are critical for division of spermatogonia and for demonstrating effects of hypophysectomy (Dobson and Dodd 1977a). ln Squa/us, a zone of degeneration appears in the testis annually in early spring (Simpson and Wardle 1967). This observation, together with the aforementioned experimental studies, can be interpreted as an indication that gonadotropin levels fluctuate seasonally as part of the normal breeding cycle. Nonetheless,
The Testis and Spermatogenesis I 77
it is important to note that animals surviving as long as two years after ventral lobectomy, although having testes markedly reduced in weight, have all spermatogenic stages present (Dobson and Dodd 1977b). One explanation is that spermatogenesis is not absolutely dependent on pituitary hormones but is rendered more 'efficient' by their presence, a concept supported by evidence in mammals as well (Vernon ct a/. 1975).
